Chlamydia psittaci should be a notifiable infectious disease everywhere
Authors: Shaoqiong Liu, Ziqi Cui, Michael J Carr, Ling Meng, Weifeng Shi, Zhenjie Zhang
Journal: The Lancet Microbe (2022)

Previously in The Lancet Microbe, Zhenjie Zhang and colleagues1Zhang Z Zhou H Cao H et al.Human-to-human transmission of Chlamydia psittaci in China, 2020: an epidemiological and aetiological investigation.Lancet Microbe. 2022; 3: e512-e520Summary Full Text Full Text PDF PubMed Scopus (23) Google Scholar described the outbreak of psittacosis in China that occurred at the end of 2020.1Zhang Z Zhou H Cao H et al.Human-to-human transmission of Chlamydia psittaci in China, 2020: an epidemiological and aetiological investigation.Lancet Microbe. 2022; 3: e512-e520Summary Full Text Full Text PDF PubMed Scopus (23) Google Scholar Psittacosis, arising from infection with the Gram-negative, obligate intracellular bacterium Chlamydia psittaci, is a global and important zoonotic disease, with numerous laboratory-confirmed cases being reported in a growing number of countries, including China, the USA, and Australia.2Tantengco OAG Gestational psittacosis: an emerging infection.Lancet Microbe. 2022; 3: e728Summary Full Text Full Text PDF PubMed Scopus (2) Google Scholar, 3McGovern OL Kobayashi M Shaw KA et al.Use of real-time PCR for Chlamydia psittaci detection in human specimens during an outbreak of psittacosis—Georgia and Virginia, 2018.MMWR Morb Mortal Wkly Rep. 2021; 70: 505-509Crossref PubMed Scopus (11) Google Scholar, 4Polkinghorne A Weston KM Branley J Recent history of psittacosis in Australia: expanding our understanding of the epidemiology of this important globally distributed zoonotic disease.Intern Med J. 2020; 50: 246-249Crossref PubMed Scopus (16) Google Scholar The clinical presentation of psittacosis frequently overlaps with that of life-threatening viral infections and so psittacosis is often overlooked in diagnostic algorithms and misdiagnosed in clinical practice. It is generally accepted that psittacosis is principally caused by human exposure to excreta from domesticated poultry and wildfowl harbouring C psittaci; however, increasing evidence1Zhang Z Zhou H Cao H et al.Human-to-human transmission of Chlamydia psittaci in China, 2020: an epidemiological and aetiological investigation.Lancet Microbe. 2022; 3: e512-e520Summary Full Text Full Text PDF PubMed Scopus (23) Google Scholar, 2Tantengco OAG Gestational psittacosis: an emerging infection.Lancet Microbe. 2022; 3: e728Summary Full Text Full Text PDF PubMed Scopus (2) Google Scholar indicates that human-to-human transmission of C psittaci is an emerging public health risk, especially for health-care workers and their close contacts (appendix). For example, in several retrospective analyses of documented outbreaks of psittacosis, 22 health-care workers were infected while treating infected patients and 20 close contacts were also subsequently infected via human-to-human transmission.1Zhang Z Zhou H Cao H et al.Human-to-human transmission of Chlamydia psittaci in China, 2020: an epidemiological and aetiological investigation.Lancet Microbe. 2022; 3: e512-e520Summary Full Text Full Text PDF PubMed Scopus (23) Google Scholar, 3McGovern OL Kobayashi M Shaw KA et al.Use of real-time PCR for Chlamydia psittaci detection in human specimens during an outbreak of psittacosis—Georgia and Virginia, 2018.MMWR Morb Mortal Wkly Rep. 2021; 70: 505-509Crossref PubMed Scopus (11) Google Scholar, 5Hughes C Maharg P Rosario P et al.Possible nosocomial transmission of psittacosis.Infect Control Hosp Epidemiol. 1997; 18: 165-168Crossref PubMed Scopus (53) Google Scholar, 6Wallensten A Fredlund H Runehagen A Multiple human-to-human transmission from a severe case of psittacosis, Sweden, Jan–Feb 2013.Euro Surveill. 2014; 1920937Crossref PubMed Google Scholar Worldwide, most patients with C psittaci infection are not strictly isolated while they are diagnosed and treated in hospital, which increases the risk of transmission and the accompanying threat to public health. Worryingly, the global clinical misdiagnosis rate of sporadic cases of psittacosis is as high as 50–80%.7Gorman J Cook A Ferguson C van Buynder P Fenwick S Weinstein P Pet birds and risks of respiratory disease in Australia: a review.Aust N Z J Public Health. 2009; 33: 167-172Crossref PubMed Scopus (10) Google Scholar This high rate of misdiagnosis is largely attributable to the non-specific nature of clinical symptoms, which overlap with those of other respiratory pathogens, and the fact that C psittaci laboratory testing is not routinely included when screening for respiratory tract infections. The US Centers for Disease Control and Prevention (CDC) have issued a case definition of psittacosis, which includes the isolation of C psittaci from respiratory or blood specimens and the detection of IgG against C psittaci by microimmunofluorescence.8Balsamo G Maxted AM Midla JW et al.Compendium of measures to control Chlamydia psittaci infection among humans (psittacosis) and pet birds (avian chlamydiosis), 2017.J Avian Med Surg. 2017; 31: 262-282Crossref PubMed Scopus (100) Google Scholar Because the heat shock proteins of C psittaci and other species of Chlamydia possess conserved epitopes, antibodies can cross-react between them, thus making it difficult to differentiate between C psittaci, Chlamydia pneumoniae, and Chlamydia trachomatis as the cause of infection.9Beeckman DS Vanrompay DC Zoonotic Chlamydophila psittaci infections from a clinical perspective.Clin Microbiol Infect. 2009; 15: 11-17Summary Full Text Full Text PDF PubMed Scopus (195) Google Scholar No current microimmunofluorescence method is specific for C psittaci; therefore, the case definition issued by the CDC has inherent limitations for the timely and reliable diagnosis of human psittacosis. Diagnosis aided by the isolation and culture of C psittaci, although highly reliable, has a low detection rate and is very time-consuming, and thus is not conducive to the rapid clinical diagnosis of psittacosis either. Therefore, the shortcomings of current testing modalities clearly pose challenges for prompt diagnosis, resulting in delayed treatment for people with psittacosis. A combination of metagenomic next-generation sequencing and species-specific real-time PCR based on the ompA gene provides a set of practical laboratory techniques from which all chlamydial infections can be routinely detected and distinguished (appendix).1Zhang Z Zhou H Cao H et al.Human-to-human transmission of Chlamydia psittaci in China, 2020: an epidemiological and aetiological investigation.Lancet Microbe. 2022; 3: e512-e520Summary Full Text Full Text PDF PubMed Scopus (23) Google Scholar, 3McGovern OL Kobayashi M Shaw KA et al.Use of real-time PCR for Chlamydia psittaci detection in human specimens during an outbreak of psittacosis—Georgia and Virginia, 2018.MMWR Morb Mortal Wkly Rep. 2021; 70: 505-509Crossref PubMed Scopus (11) Google Scholar, 10Geens T Dewitte A Boon N Vanrompay D Development of a Chlamydophila psittaci species-specific and genotype-specific real-time PCR.Vet Res. 2005; 36: 787-797Crossref PubMed Scopus (65) Google Scholar These molecular diagnostic approaches can rapidly, accurately, and sensitively detect C psittaci nucleic acids in different sample types and we recommend that they be routinely included in the clinical diagnosis of psittacosis, especially for patients with severe pneumonia in intensive care units. Although psittacosis is a notifiable disease and must be reported to authorities within 48 h in some countries, most UN-defined developing countries, such as China and India, have not yet listed psittacosis as a notifiable infectious disease. Current global psittacosis case numbers might be greatly underestimated. For example, since 2018, there have been fewer than 100 cases of psittacosis per year in the USA, as reported by the CDC. The underestimation of case numbers could be explained by asymptomatic transmission, a paucity of awareness among doctors as to the potential risk of human-to-human transmission, and the aforementioned clinical and laboratory testing issues, resulting in people with psittacosis who do not present with pneumonia often going unnoticed. Therefore, public health departments should raise awareness of psittacosis, formulate preventive measures for groups at high risk of infection, validate and administer rapid molecular techniques for diagnosis, and strictly implement isolation measures for patients with psittacosis so as to avoid the occurrence of nosocomial infections and family clusters of infections. Given the potential biosafety threat of C psittaci, we call for the routine inclusion of C psittaci when screening for respiratory pathogens, particularly in populations at high risk. Notably, we propose that psittacosis should be included in the list of notifiable infectious diseases in China and elsewhere as soon as possible, so as to better estimate the real burden of psittacosis and to prevent and control this zoonosis. We declare no competing interests. This work was supported by the Academic Promotion Programme of Shandong First Medical University (2019QL006), the Medical and Health Science and Technology Development Program of Shandong Province, China (202001060452), and the Shandong Traditional Chinese Medicine Science and Technology Development Plan, China (Q-2022139). LM, WS, and ZZ contributed equally to this Comment. Download .pdf (1.69 MB) Help with pdf files Supplementary appendix Chlamydia psittaci should be included in veterinary legal quarantine everywhereWith great interest we read the Comment published in The Lancet Microbe by Shaoqiong Liu and colleagues.1 We strongly agree with their argument. In the past 10 years, human cases of Chlamydia psittaci infection have increased in several countries, especially in China.2–4 Generally speaking, C psittaci infection results from having close contact with an infected animal or exposure to excreta from animals harbouring C psittaci. However, C psittaci infection has been frequently reported in health-care workers from human-to-human transmission. Full-Text PDF Open Access